Add conf.d directory with placeholder

conf.d/README.md

@@ -0,0 +1,13 @@
+# Site Configurations
+Put your server blocks (vhosts) in this directory.
+Example: `my-site.conf`
+
+```nginx
+server {
+    listen 80;
+    server_name example.com;
+    location / {
+        proxy_pass http://internal:8080;
+    }
+}
+```